Electrochromism describes the induction of a color change in a medium as a result of charge transfer or electron transfer caused by an externally applied potential. The color changes are indications of induced chemical changes in the species of interest. For most chemical species exhibiting this effect, the change is from one color to another. As an example, viologen dye molecules change from yellow-orange to blue when reduced at a cathode. J. Bruinik, C. G. A. Kregting, and J. J. Ponjee, J. Electrochem. Soc. 124, 1853 (1977). Solid films of WO.sub.3 also show electrochromism with transparent films becoming blue upon reduction.
In order for electrochromic materials to be useful for display purposes, they must have optical absorption in the visible spectrum and exhibit mixed conduction capability (i.e. electronic and ionic). It is also highly desirable to exhibit high contrast from the background in order to modulate ambient light. Electrochromic materials generally have these properties. Electrochromic materials are usually operated with low voltages and can provide suitable contrasts with charge transfer of only several millicoulombs of electrical charge per square centimeter of display area. Erasure is easily made by polarity changes. These materials may also have the ability to hold images for the required response time of the human eye (about 0.1 second) and this further may allow for the use of memory effects. A major disadvantage of electrochromic displays is the lifetime of the device. Chemical degradation frequently occurs as usage time increases.
The most studied systems which utilize the electrochromic effect are displays based on WO.sub.3. B. W. Faughnan, Topics in Applied Physics: Display Devices, Volume 40, J. I. Pankove (ed.,), Springer-Verlag, New York, (1980), p. 181. Amorphous films of WO.sub.3 have high ion mobilities as necessary and exhibit coloring and bleaching between blue and transparent colors. The device lifetime is extremely sensitive to the presence of oxygen and water. To date, a commercial viable system based on the oxide films has yet to be produced.
Organic species have also been examined as an alternative but frequently lack the desired contrast since they convert between two distinct colors and do not have a transparent form. J. Bruinick, C. G. A. Kregting, and J. J. Ponjee, J. Electrochem. Soc. 124, 1853 (1977). M. M. Nicholson and F. A. Pizzarello, J. Electrochem, Soc. 127, 821 (1980).
Polyaniline is the chemical name given to the product of anodic oxidation of aniline. The formation of polymeric compounds by oxidation of aniline has been known for some time. S. Venkataraman, Chemistry of Synthetic Dyes, Volume II, Academic Press, New York (1952), p. 772. The products are highly colored films or solids. The first modern electrochemical study of this oxidation at solid electrodes was carried out by Adams and co-workers. D. M. Mohilner, R. N. Adams, and W. J. Argersinger, Jr., J. Am. Chem. Soc. 84, 3618 (1962). A polymeric product was obtained which they suggested to be an octamer of head to tail para coupling of aniline monomers. ##STR1## This octamer was prepared in sulfuric acid electrolyte. It has been suggested to be emeraldine sulfate, a highly colored salt which had been observed in previous studies involving chemical oxidation. Since their original paper, Adams and coworkers have acknowledged that other coupling modes (i.e. head-to-head or tail-to-tail) are possible. J. Bacon and R. N. Adams, J. Am. Chem. Soc. 90, 6596 (1968). Although the polymeric nature of this oxidation product has been suggested for many years, the full characterization has remained inconclusive. Renewed interest in the structural nature of the polymer has been generated by recent findings of its good electrical conductivity. A. F. Diaz and J. A. Logan, J. Electroanal. Chem. 111, 111 (1980). They noted that the polymer is conducting in both anodic and cathodic regions. They also noted that the film color can be altered by varying the electrode potential.
A more recent article has presented a brief spectral characterization of films grown on indium oxide electrodes. T. Kobayashi, H. Yaneyama, and H. Tamura, J. Electroanal. Chem. 161, 419 (1984).
Our invention is in part based on the fact that polyaniline films are conducting and although it has not been established, it is believed that there are ionic and electronic contributions to its conductive properties. It is expected that relatively high ion mobilities (particularly proton) are found for this films. The films are prepared in aqueous solution and do not dissolve. They are also relatively stable toward oxygen.
Our prior invention embodied an electronic display element useful in electronic color display devices. Broadly, that invention comprised two electrodes, at least one electrode being transparent, having electrolyte disposed therebetween. A thin film of polymeric aniline or its chemical derivatives was placed in electrical communication with at least one of said electrodes. In the preferred embodiment, the polymeric film was coated electrolytically on the anode using an acidic solution containing the monomeric aniline. After the polymer film was coated, the solution was replaced by an acidic electrolyte solution which did not contain aniline monomer. Applying different voltages across the interface between the polymer film and the electrolyte resulted in color changes of the film. Color changes achieved included blue, green, yellow and transparent. The color changes were sharp and distinct and repeated cycling of the voltage did not cause degradation of the film and the response time of the color change was short.
The display element of that invention overcomes the prior art problems of longevity and the prior art problems of the inability of the films to repeatedly produce color changes, including transparent, which are necessary for successful application of electrochromism in electronic color display devices. Further advantages of that invention were a display screen in a thin plate or rollable sheet which consumes a minimal amount of electrical power. Further, the area of the display device can be very large in reference to the physical limitations imposed on the presently available cathode-ray tubes. Most importantly, a multicolor display was achieved which capability is not available in present liquid crystal display devices.
Although our prior invention embodied and embraced the use of the color changes within two transparent surfaces, the present invention is directed more particularly to the use of the polymeric aniline between two transparent surfaces for use such as in windows, windshields, glasses, bowls, decorative panels and the like.
There are presently available `tinted` or `color changeable` transparent panels such as sunglasses and there are photochromic glasses which respond to light to change the color of those glasses from a first shade of the color to a second shade or tint of the color; i.e. from dark brown to light brown.
The present invention embodies a multi-color switchable panel while the prior art photochromic panels have only one color. The panel of our invention responds rapidly while the photochromic prior art glasses take minutes to change its transmittency and color. The polychromic panels of the present invention cannot only change colors and tints with rapidity but can also become completely transparent.
While it is understood that prior art glasses such as so called photosensitive sunglasses change shades a typical time to change between a light transmittency of 20% to 80% requires 120 to 180 seconds. In our invention to effect the same change in transmittency requires a time of only 0.05 to 1 seconds. Thus, our invention includes the ability to change between colors as defined previously and to change between tints or shades within a color within a very rapid time frame which is not believed achievable in the prior art and/or to change within tints and to change to transparent which is not believed achievable in the prior art.